Table of Contents
Logo commands, or primitives, and system names, or variables, are the basic building blocks of Logo programs and control of the Logo environment. Use Logo primitives to move the turtle, control the appearance of your computer screen, and to write procedures for your Logo projects. This chapter provides a brief definition of each Logo primitive and variable as a guide for Logo programming.
Logo Language Overview
The information does not replace a thorough tutorial. It does not even Please see our own Getting Started tutorial for more details. Brian Harvey’s excellent three-volume book set “Computer Science Logo Style” is available as a free download on his website at https://www.cs.berkeley.edu/~bh. This is highly recommended reading if you would like to become more familiar with Logo.
Logo is an untyped language; variables and procedure parameters may
contain any data type. Logo numbers are in 64-bit floating point format
with up to 15 digits of decimal precision. Strings are 8-bit encoded;
Unicode is not supported. Booleans are represented with the constants
FALSE. Logo converts these data types to the required types
The main data type is the list. Several commands are available for list access and manipulation. Arrays are separate objects with their own commands.
Turtles, bitmaps, arrays, and controls are property lists. You can, for example, read or set a turtle’s position with property list commands. Some properties are callable; these properties correspond to C++ methods. Use commands like ASK or CPROP to execute these properties.
Numbers: Numbers are floating-point. Use the prefix #B, #O, #D and #H to enter binary, octal, decimal, and hex values like, for example, #HFA or #B1101.
Strings: A single leading double quote defines a string constant
“HELLO. These constants are converted to upper case unless the
:CASE variable is set to FALSE. Strings
enclosed in vertical bars are not converted; such a string constant
needs to be quoted like
“|Hello|. You can use backquotes to combine
vertical bars and the double quote as in `Hello`.
Lists: Lists constants are enclosed in square brackets as in [HELLO |Joe|]. Note the second constant, which is a mixed-case string. No double quote is necessary.
The MAKE command stores a value into a
variable as in
MAKE “NAME “JACK. A single colon in front of a name
loads the variable. Example:
MAKE "CENTER [0 0] SETXY :CENTER
Variables are global except for procedure parameters and local variables, which need to be declared inside a procedure with the LOCAL command. Procedures have access to all variables in the calling procedure chain.
The () parentheses have a special meaning in Logo:
- If used with a procedure call, Logo evaluates all arguments and
feeds them into the procedure which is the list’s first element.
This makes it possible to call a procedure with a variable list of
arguments. The PRINT command, for example,
takes one argument, but if used with parentheses,
PRINT accepts any number of arguments.
(PRINT)prints a newline only, while
(PRINT “A “B :CENTER)prints three values and a newline.
- In arithmetic expressions, parentheses are used to group expressions as in any other programming language.
The TO command defines a Logo procedure. The formal
parameters (called “inputs” in Logo) are written as variables. the word
END ends a procedure declaration. Procedures are lists; they can be
declared dynamically with the DEFINE command, or
extracted with the TEXT command. Self-modifying
code is, therefore, possible.
A procedure input is written as
:NAME. Optional inputs are written as
two-element lists as in [:NAME value]. Note that Terrapin Logo does
not evaluate optional inputs. Optional inputs must follow regular
inputs. If the last input is single-element list like [:NAME], it acts
as a catch-all input that collects all remaining inputs as a list.
Finally, a procedure declaration may have a number as its last input
that defines the number of default inputs. If no number is given, that
number is the number of regular inputs.
TO FOO :NAME [:VALUE 5] [:REST]
Defines a procedure
FOO with a regular input, a second, optional input
that defaults to the value 5, and a last input that collects remaining
FOO is used without parentheses, it needs one input; it
requires at least one input.
FOO 1 :NAME = 1 :VALUE = 5 :REST =  (FOO 1 2 3 4) :NAME = 1 :VALUE = 2 :REST = [3 4]
Recursion is a basic Logo concept; use it whenever possible. Logo detects tail recursion properly.
Logo offers the usual set of program flow commands like IF, REPEAT, FOR, or WHILE. The FOREACH command iterates of a list or a word, and the TELL, ASK, and EACH commands iterate over turtles, bitmaps, and controls.
Input and Output
Terrapin Logo supports files and folders. The OPEN command opens a file and outputs a channel number. You can assign this channel number to the global variables :STANDARD.INPUT and :STANDARD.OUTPUT to make all input/output commands work with this channel number, or you can supply the channel number to most I/O commands as an additional, optional input.
This list of Logo primitives and system names is organized by subject area. Commands that work in the same area, such as controlling the Logo turtle, are grouped together. Within each topic area commands are again organized by function, such as moving the turtle or changing the turtle’s state. Within each subheading the commands are listed alphabetically. If you are interested in a particular function of Logo, you can go to the topic area and review the commands that are useful for achieving that purpose.
The command listing is in two columns. The first column provides the syntax for the command with its name followed by any required inputs. Syntax for primitives that are often used with optional inputs is also listed. Note that when Logo primitives that are used with more or fewer than the default number of inputs, both the primitive and its inputs must be enclosed in parentheses as illustrated. The second column provides the abbreviation for the Logo command, if any. The full name and the abbreviation may be used interchangeably for any Logo primitives that have abbreviations. The following paragraph provides a brief definition of the function of the primitive or system name.
The following words are used in the definitions of the Logo primitives and system names that are listed in this section (a dotted underline indicates the term is in this list):
|:||precedes a system name or variable to access it, and is equivalent to THING|
|”||precedes a special Logo word, such as a name|
|#||precedes a command that changes the base of a number|
|…||indicates that any number of similar arguments may follow|
|bitmap||graphics image that may be stored on disk or on the clipboard|
|character||an alphanumeric character|
|channel||an input or output stream identification number|
|color||a color number, name, or 3-element RGB list|
|command||one or more instructions to Logo|
|control||a graphical Logo object used to display or get information, such as a checkbox, text edit field or static text.|
|current entity/turtle||the first entity/turtle in the TELL list|
|descr||a word containing file type descriptors|
|entity||a Logo object that can be placed in a Graphics window, such as a turtle, bitmap or control|
|expression||a statement whose output could be a word, a list, a number, or TRUE or FALSE|
|filename||the name of a disk file|
|identifier||a word, either a number or a name, that identifies an object|
|integer||a whole number|
|list||an ordered set of Logo objects surrounded by brackets|
|mode||format for file reading and writing|
|name||word naming a procedure, variable, or other Logo object|
|number||an integer or decimal number|
|object||a Logo object, such as a word, list, procedure, window or turtle|
|objecttype||the name of a Logo object type|
|port||an input/output port|
|primitive||name of a Logo primitive (a built-in procedure)|
|procedure||name of a Logo procedure|
|property||name of a Logo property|
|shape||a turtle shape name|
|TRUE||Logo true object|
|FALSE||Logo false object|
|[x y]||a list specifying Cartesian x and y coordinates, or width/height|